1. Art Field
The present invention relates generally to a phosphor substance used for EL (electroluminescence) devices, and more particularly to an EL phosphor multilayer thin film and an EL device.
2. Background Art
In recent years, thin-film EL devices have been increasingly studied for compact or large yet lightweight flat display panels. A monochromatic thin-film EL display using a fluorescent thin film comprising manganese-added zinc sulfide for yellowish orange light emission has already been practically used in the form of a double-insulation structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. Referring to FIG. 2, a lower electrode 5 is formed in a given pattern on a glass substrate 1, and a first insulating layer 2 in the form of a dielectric thin film is provided on the lower electrode 5. On the first insulating layer 2, there are provided a light-emitting layer 3 and a second insulating layer (dielectric thin film) 4 in this order. An upper electrode 6 is formed on the second insulating layer 4 in such a given pattern as to form a matrix with the lower electrode 5. Usually, the phosphor thin film has been annealed at a temperature lower than the strain point of the glass substrate for luminance improvements.
Recently, a structure using a ceramic substrate for the substrate 1 and a thick-film dielectric layer for the insulating layer 2 has also been put forward. With this structure wherein a ceramic material such as alumina is used as the substrate, it is possible to anneal the phosphor thin film at high temperature, thereby achieving luminance improvements. Because the thick-film dielectric layer is used for the insulating layer, this structure also features the ability to provide a panel having higher resistance to dielectric breakdowns and higher reliability than does an EL device using a thin film for the insulating layer.
To accommodate well to personal computer displays, TV displays and other displays, color displays are absolutely needed. Thin-film EL displays using a sulfide fluorescent material thin film are satisfactory in reliability and resistance to environmental conditions. At present, however, they are thought of as being unsuitable for color display purposes, because the properties of an EL fluorescent material for emitting the three primary colors or red, green and blue are less than satisfactory. Candidates for a blue emitting fluorescent substance are SrS:Ce where SrS is used as a matrix material and Ce as a luminescent center, SrGa2S4:Ce and ZnS:Tm, candidates for a red emitting fluorescent substance are ZnS:Sm and CaS:Eu, and candidates for a green emitting fluorescent substance are ZnS:Tb, CaS:Ce, etc., and studies thereof are now under way.
These phosphor thin film for emitting the three primary colors, viz., red, green and blue are poor in light emission luminance, efficiency, color purity, etc., and so color EL panels are still on impractical levels. For blue in particular, relatively high luminance is obtained using SrS:Ce. However, such luminance is still unsatisfactory for blue applied to full-color displays, with color purity shifted to a green side. Thus, much improved blue emitting layers are in great demand.
To provide a solution to these problems, thiogallate or thioaluminate blue fluorescent substances such as SrGa2S4:Ce, CaGa2S4:Ce and BaAl2S4:Eu are now under development, as set forth in JP-A""s 07-122364 and 08-134440, Shingaku Giho EID98-113, pp. 19-24, and Jpll. J. Appl. Phys. Vol. 38, (1999), pp. L1291-1292.
The inventors, too, have made studies of thioaluminate blue fluorescent substance on the premise that blue EL materials of high luminance are essentially required for the achievement of full-color EL panels. However, the resultant luminance is at most 100 cd/m2 on 1 kHz driving and so is still less than practical.
To provide a solution to the aforesaid problems, there is an increasing demand for a fluorescent substance capable of emitting light with high luminance, especially a blue phosphor thin-film material. An object of the invention is to provide an EL phosphor multilayer thin film capable of emitting light with enhanced luminance and an EL device.
Such an object is achievable by any one of the following embodiments (1) to (8) of the invention.
(1) An EL phosphor multilayer thin film, wherein:
a phosphor thin film and a dielectric thin film are stacked one upon another,
said phosphor thin film comprising a matrix material containing as a main component at least one compound selected from an alkaline earth thioaluminate, an alkaline earth thiogallate and an alkaline earth thioindate, and an rare earth element as a luminescent center, and
said dielectric thin film comprising an alkaline earth oxide.
(2) The EL phosphor multilayer thin film according to (1) above, wherein said matrix material is a barium thioaluminate.
(3) The EL phosphor multilayer thin film according to (1) above, wherein said rare earth element is Eu.
(4) The EL phosphor multilayer thin film according to (1) above, wherein said alkaline earth oxide is a perovskite oxide.
(5) The EL phosphor multilayer thin film according to (1) above, wherein said alkaline earth oxide is barium titanate.
(6) The EL phosphor multilayer thin film according to (1) above, wherein said dielectric thin film has a specific dielectric constant of 100 or greater.
(7) The EL phosphor multilayer thin film according to (1) above, wherein said dielectric thin film has a thickness of 100 nm or greater.
(8) An EL device comprising an EL phosphor multilayer thin film as recited in (1) above.
The present invention has been accomplished in the process of phosphor thin-film formation experiments made with a view to bringing the luminance of a thioaluminate blue phosphor substance up to a practical level. The thus achieved EL phosphor multilayer thin film makes a breakthrough in light emission luminance over a conventional EL phosphor thin film.